It seems that one implementation went into the trunk (tested: r2368):
# things to overcrowded destinations won't loaf if active (default off)
avoid_overcrowding = 0
I used an older, well-developed savegame with avoid_overcrowding=1 (also seperate_halt_capacities=1, discussed in this same topic), and found the change to work (detail: people who want to go exactly to the overcrowded station, seem to travel even in that case), but the overall result is rather unsatisfactory (in my opinion).
For example, I looked at two major backbone nodes, both a little(!) overcrowded: most of the time, nearly empty trains run between the two, because the people from the first node won't travel to the second, as it is already too full, and vice versa. This is a kind of deadlock, which in itself might eventually become resolved over a longer time. However, the leaf nodes of the network keep filling up with people waiting for the upper nodes to clear, so whenever those leave overcrowded state, they will return to it immediately, and the deadlock continues after a short interruption.
(One alternative suggestion in this topic had been to not route across full nodes, so alternative routes would first be used, and after that fails, people would decline to travel over the player's network at all.)
(edit: removed claim that this was my idea)